1. Field of the Invention
The present invention relates to an arrayed waveguide grating (AWG) type optical multiplexer/demultiplexer which is employable as a wavelength-selecting device in a wavelength division multiplexing (WDM) transmission system.
2. Related Background Art
AWG type optical multiplexer/demultiplexers (hereinafter referred to as AWG circuits) are widely in use as a wavelength filter, which can take out or insert a specific wavelength upon interference, for a wavelength-selecting device in WDM transmission systems. Also, since the AWG circuits can be realized by general fine processing procedures such as lithography or etching without necessitating the machining as precise as that of diffraction gratings or the forming of multilayer films as precise as that of interference films, they are expected to develop as a main optical device in future WDM transmission systems together with their capability of assembling with other optical waveguide devices.
Such an AWG circuit has a structure in which an input waveguide, an input slab waveguide, a plurality of channel waveguides having respective lengths different from each other (phased array), an output slab waveguide, and a plurality of output waveguides are integrally formed on a single substrate and are covered with cladding glass.
In particular, U.S. Pat. No. 5,002,350 discloses an optical multiplexer/demultiplexer in which, among a plurality of channel waveguides, those adjacent each other have a constant optical path length difference therebetween. At respective portions where the channel waveguides connect with the input and output slab waveguides, in order to improve the wavelength multi/demultiplexing performances, the end parts of channel waveguides are arranged at equally spaced intervals along an arc having a predetermined radius or each of the end faces of input and output slab waveguides to which the end parts of channel waveguides are connected at equally spaced intervals is processed so as to match an arc having the predetermined radius.
The inventors have studied the conventional optical multiplexer/demultiplexers and, as a result, have found problems as follows. Namely, if the end parts of channel waveguides are arranged like an arc, then the convergence of light outputted from those located in peripheral portions to the slab waveguides may deteriorate under the influence of aberration and the like, while the convergence of light outputted from those located near the center thereof is fully maintained. In addition, if the end parts of channel waveguides are arranged at equally spaced intervals along an arc, then the convergence of light outputted from the channel waveguides located in peripheral portions in particular to the slab waveguides will further deteriorate due to interactions between adjacent channel waveguide. As a result, among the output waveguides provided so as to correspond to respective signal channels in the conventional optical multiplexer/demultiplexers, crosstalk between adjacent signal channels may be remarkable in the output waveguides located in the periphery in particular, whereas there has been a technical limit to lowering the crosstalk.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical multiplexer/demultiplexer comprising a structure which can effectively lower the crosstalk between adjacent signal channels among output waveguides provided so as to correspond to respective signal channels, in the output waveguides located in the periphery in particular, thereby realizing excellent wavelength multi/demultiplexing characteristics.
The optical multiplexer/demultiplexer according to the present invention is an AWG type optical multiplexer/demultiplexer, employable as a wavelength-selecting device in a WDM transmission system, comprising a substrate, and at least one input waveguide, a first slab waveguide, n (xe2x89xa73) channel waveguides, a second slab waveguide, and a plurality of output waveguides provided for respective signal channels, which are disposed on the substrate.
In the optical multiplexer/demultiplexer according to the present invention, the first and second slab waveguides have respective predetermined slab lengths. In general, a slab length corresponds to the focal length of the optical input end functioning as the lens surface of the respective slab waveguide. The input waveguide is a waveguide for guiding to the first slab waveguide individual signals having respective channel wavelengths set at predetermined wavelength intervals as signal channels, and has an output end optically connected to an optical input end face of the first slab waveguide. The n channel waveguides are waveguides having lengths different from each other, and are two-dimensionally arranged on the substrate while in a state where an optical input end of each channel waveguide is optically connected to an optical output end face of the first slab waveguide so as to sandwich the first slab waveguide together with the input waveguide whereas an optical output end of each channel waveguide is optically connected to an optical input end face of the second slab waveguide so as to sandwich the second slab waveguide together with the output waveguides. The output waveguides are waveguides two-dimensionally arranged on the substrate while in a state where respective optical input ends thereof are optically connected to an optical output end face of the second slab waveguide, and are used for separately taking out signals having respective channel wavelengths set at predetermined wavelength intervals.
In particular, in the optical multiplexer/demultiplexer according to the present invention, at least one of the optical output end face of the first slab waveguide and the optical input end face of the second slab waveguide each connected to the n channel waveguides is processed flat so as to extend along a line intersecting the n channel waveguides. As a consequence, among the n channel waveguides, those adjacent each other have optical path length differences different from each other.
Specifically, with respect to the average value obtained from respective optical path length differences between all adjacent channel waveguides in the n channel waveguides, it is preferred that the maximum deviation of optical path length difference between adjacent channel waveguides in the n channel waveguides be set to 3% or more. It means that, letting xcex94Lk (k=1 to (nxe2x88x921)) be each optical path length difference between adjacent channel waveguides, xcex94LMAX be the maximum optical path length difference (or minimum optical path length difference) between adjacent channel waveguides, and xcex94LAVE be the average value of optical path length difference, at least the deviation xcex7 (maximum deviation) of maximum optical path length difference xcex94LMAX with respect to the average value xcex94LAVE satisfies the following condition:   η  =                    |                              Δ            ⁢                          xe2x80x83                        ⁢                          L                              A                ⁢                                  xe2x80x83                                ⁢                V                ⁢                                  xe2x80x83                                ⁢                E                                              -                      Δ            ⁢                          xe2x80x83                        ⁢                          L              MAX                                      |                    Δ        ⁢                  xe2x80x83                ⁢                  L                      A            ⁢                          xe2x80x83                        ⁢            V            ⁢                          xe2x80x83                        ⁢            E                                ≥    0.03  
where       Δ    ⁢          xe2x80x83        ⁢          L              A        ⁢                  xe2x80x83                ⁢        V        ⁢                  xe2x80x83                ⁢        E              =                              ∑                      k            =            1                                n            -            1                          ⁢                  Δ          ⁢                      xe2x80x83                    ⁢                      L            k                                      n        -        1              .  
Here, the optical multiplexer/demultiplexer according to the present invention is designed such that, though adjacent channel waveguides have optical path length differences different from each other, effective optical path length differences become constant between optical paths extending from the center of optical input end face of the first slab waveguide to the center of optical output end face of the second slab waveguide by way of the channel waveguides adjacent each other in order to realize wavelength multi/demultiplexing functions as a whole. Namely, as shown in FIG. 4, letting L(m) be the physical optical path length from the center P1 of optical input end face of the first slab waveguide to the center P2 of optical output end face of the second slab waveguide by way of the m-th (2xe2x89xa6mxe2x89xa6n) channel waveguide, neff(m) be the effective refractive index of the m-th channel waveguide, L(mxe2x88x921) be the physical optical path length from the center P1 of optical input end face of the first slab waveguide to the center P2 of optical output end face of the second slab waveguide by way of the (mxe2x88x921)-th channel waveguide, and neff(mxe2x88x921) be the effective refractive index of the (mxe2x88x921)-th channel waveguide, the integrated value of product of physical optical path length and effective refractive index along optical paths from P1 to P2 satisfies the following condition:                     ∫        P1        P2            ⁢                                    L            ⁡                          (              m              )                                ·                                    n                              e                ⁢                                  xe2x80x83                                ⁢                ff                                      ⁡                          (              m              )                                      ⁢                  ⅆ          x                      -                  ∫        P1        P2            ⁢                                    L            ⁡                          (                              m                -                1                            )                                ·                                    n                              e                ⁢                                  xe2x80x83                                ⁢                ff                                      ⁡                          (                              m                -                1                            )                                      ⁢                  ⅆ          x                      =  constant
between the m-th and (mxe2x88x921)-th channel waveguides adjacent each other as a wavelength multi/demultiplexing condition in the optical multiplexer/demultiplexer. Here, among the n channel waveguides, the optical path length difference xcex94Ln between those adjacent each other is given by the following expression:       Δ    ⁢          xe2x80x83        ⁢          L      0        -            f      0        ·          (              1        -                  1                      cos            ⁢                          xe2x80x83                        ⁢                          θ              P                                          )      
where
xcex94L0 is the theoretical value of the maximum optical path length difference for enabling the channel waveguides to function as a diffraction grating;
f0 is the maximum distance between the center of optical input end face of first slab waveguide to the optical input ends of channel waveguides or the maximum distance between the optical output ends of channel waveguides to the center of optical output end face of second slab waveguide; and
xcex8P is the angle formed between the P-th (Pxe2x88x921, 2, . . . , n) channel waveguide and a normal of the optical output end face of first slab waveguide or optical input end face of second slab waveguide.
As mentioned above, the optical multiplexer/demultiplexer according to the present invention is designed such that, while channel waveguides adjacent each other are allowed to have optical path length differences different from each other, optical paths traveling by way of respective channel waveguides adjacent each other have a constant effective optical path length difference as the optical paths including the slab waveguides in total. It means that the structure for connecting channel waveguides to the flat connecting end face (at least one of the optical output end face of first slab waveguide and the optical input end face of second slab waveguide) can be changed arbitrarily without being restricted by multi/demultiplexing conditions. As a result, it becomes easier to design the arrangement of channel waveguides, and their layout attains a higher degree of freedom, which makes it possible to design a structure for effectively lowering the crosstalk between adjacent signal channels in output waveguides located in the periphery in particular among the output waveguides provided so as to correspond to respective signal channels.
In order to adjust the focal position in the first slab waveguide in the optical multiplexer/demultiplexer according to the present invention, it is preferred that the channel waveguides connected to the optical output end face of the first slab waveguide be arranged such that the optical input ends thereof are directed to the center of optical input end face of the first slab waveguide. Also, in order to adjust the focal position in the second slab waveguide, it is preferred that the channel waveguides connected to the optical input end face of the second slab waveguide be arranged such that the optical output ends thereof are directed to the center of optical output end face of the second slab waveguide. Namely, with respect to at least one flat end face of the optical output end face of first slab waveguide and the optical input end face of second slab waveguide, the channel waveguides connected to this flat end face form respective angles different from each other in the optical multiplexer/demultiplexer according to the present invention. In other words, the channel waveguides are arranged on the substrate such that, among tip portions thereof including the optical input ends, those adjacent each other have intervals different from each other. Also, the channel waveguides are arranged on the substrate such that, among tip portions thereof including the optical output ends, those adjacent each other have intervals different from each other.